Many models of gas turbine engines use one or two reduction stages of epicyclic gearing. An epicyclic reduction stage typically comprises a sun gear which receives an input torque, two or more planet gears which are intermeshed with, and revolve around the sun gear, receiving torque therefrom, and a carrier. The carrier generally has two functions firstly, it carries the planet gears as they revolve collectively around the sun gear, and secondly it transfers the torque received from the revolving movement to an output shaft. The planet gears are free to rotate around their axes within the carrier, but the circular movement of their axes around the sun gear drives the rotary movement of the carrier. The planet gears are typically further intermeshed and contained within a ring gear which is fixed relative to the gas turbine engine. A single reduction stage, or a first reduction stage of a double reduction stage gearbox, may receive the input torque directly from a low pressure shaft, whereas a second reduction stage receives the input torque from the first reduction stage.
One challenge in epicyclic gearing of gas turbine engines is to keep the axes of the planet gears, or more specifically of the bearings thereof, parallel to the axis of the sun gear. Indeed, the planet gears are typically received on the carrier via cylindrical bearings which may wear quickly when operated with the axes of the planet gears inclined off parallel relative to the axis of the sun gear. Spherical bearings allow for such inclination, but they are typically heavier and bulkier than cylindrical bearings, which is especially undesirable in aircraft equipment. When large torque levels are present, axial torsion can occur in the carrier which can cause displacement of one axial end of each bearing relative to the bearing's opposite axial end, and thus undesirable inclination of the bearing axis relative to the sun gear axis. Such axial torsion can be present, for instance, when the torque is transferred primarily from one axial end of the planet gear bearings.
In the context of gas turbine engines, this challenge must be faced while taking a number of other factors into consideration, such as the control of the weight of the assembly, structural integrity, durability, manufacturability and manufacturing costs. Moreover, epicyclic gearing typically requires a supply of oil a) to the interface between the sun gear and the planet gears and b) to the bearings of the planet gears. While oil supply channels can be provided within some gas turbine engine components to convey oil from an oil source to an oil outlet, this approach is tributary to the presence of an oil source and the impact of the presence of oil supply channels on structural integrity must be taken into consideration. The conveying of lubrication oil is further complicated by the fact that in gas turbine engines, some components may be designed to move, within a certain extent, relative to others to account for load distribution, and that flexible joints are often used to connect a shaft to an epicyclic gear stage, or epicyclic gear stages to one another, to accommodate such relative movement. Flexible joints typically cannot be provided with integrated oil supply channels, forcing designers to find another path for the conveyance of lubrication oil.
While these challenges had been addressed to a certain extent, in certain circumstances, there remained room for improvement.